I'm not sure about the Outback, but when I took the seats out of the WRX to haul parts back in it, the back seat weighed very little. It was almost all foam. Take a look though. I think for the WRX it was like 4 - 6 bolts and both the seat and the back came out, easy.

Now, on the other hand, the front passenger seat was pretty heavy. Not super heavy, but I'd estimate maybe 60 - 80 lbs? But then you can't take along a passenger to scare if you take that out  

I'd usually take everything out of the trunk at the rally-x. The donut + tools + jack weighed a bit.

We just did a thorough weight reduction but to a more extreme extent than you would like.
Definitely as has been said, spare tire jack etc.
-Floor mats,
-Rear seats
-Run 5L of gas
-Smaller battery or if you want to get sketchy start the car with the battery out of it and hope you don't stall.
-Run winters instead of gravels since they're lighter. Of course you need to in the winter anyways.

I know it's a little different but a steely with a gravel tire, jack, x wrench, and cordless impact cost 65lbs.